1. Field of the Invention
This invention relates to an optical splitter that is applied to, for example, a device for splitting a signal light beam in optical communications and the like.
2. Description of the Related Art
For example, in a device for optical integrated circuits used for optical communications, a so-called Y-split waveguide is known as one means for splitting a signal light beam. A splitter capable of asymmetrically splitting optical power at a ratio of 60 to 40 or a ratio of 90 to 10 is also demanded.
For example, a prior art optical splitter as shown in FIG. 14 has a main waveguide 1, a tapered waveguide 2, and a pair of split waveguides 3 and 4. The widths W3 and W4 of the split waveguides 3 and 4 are caused to differ from each other to allow optical power to be split asymmetrically. The tapered waveguide 2 tapers and expands from the main waveguide 1 toward the split waveguides 3 and 4, i.e., from width W1 to width W2. A split portion end face 5 of width Wg is formed between the split waveguides 3 and 4.
In the prior art optical splitter shown in FIG. 14, the width W2 of the tapered waveguide 2 at the connecting portions of the tapered waveguide 2 and the split waveguides 3 and 4 is equal to the sum of the widths W3 and W4 of the split waveguides 3 and 4 and the width Wg of the split end face 5. In other words, the optical splitter is designed to have the following relationships: W2=(W3+W4+Wg) and W3≠W4.
FIG. 15 shows a relationship between a ratio between the widths W3 and W4 of the split waveguides 3 and 4 and each of a split ratio and a split excessive loss in the prior art optical splitter shown in FIG. 14. In FIG. 15, the horizontal axis represents a ratio between the widths W3 and W4 of the split waveguides 3 and 4 [W3/(W3+W4)]. The left vertical axis represents a split ratio between the split waveguides 3 and 4, and the right vertical axis represents a split excessive loss. The sum of the widths W3 and W4 of the split waveguides 3 and 4 is 10 μm, and the section of the main waveguide 1 is a 7 μm×7 μm square. The relative refractive index difference between a core and a clad layer is 0.45%, the width Wg of the split portion end face 5 is 3 μm, the length of the tapered waveguide 2 is 600 μm, and the wavelength of an optical signal is 1.55 μm.
On the other hand, Jpn. Pat. Appln. KOKAI Publication No. 9-80244 describes an optical splitter wherein the positions of a pair of split waveguides are displaced in a width direction of a tapered waveguide. This prior art is directed to an optical splitter as shown in FIG. 16. The optical splitter has a pair of split waveguides 3′ and 4′ whose widths W5 are equal to each other, and the central axes of the split waveguides 3′ and 4′ are displaced by ΔX in the width direction of a tapered waveguide 2 from the central axes of the main waveguide 1 and tapered waveguide 2.
FIG. 17 shows a relationship between the amount of axial displacement ΔX and each of a split ratio and a split excessive loss in the prior art optical splitter shown in FIG. 16. In FIG. 17, the horizontal axis represents the amount of axial displacement ΔX. The left vertical axis represents the split ratio between the split waveguides 3′ and 4′, and the right vertical axis represents the split excessive loss. The section of each of the main waveguide 1 and split waveguides 3′ and 4′ is a 7 μm×7 μm rectangle. The relative refractive index difference between a core and a clad layer is 0.45%, the width Wg of a split portion end face 5 is 3 μm, the length of the tapered waveguide 2 is 600 μm, and the wavelength of an optical signal is 1.55 μm.
The prior art optical splitter shown in FIG. 14 has the drawback that the split excessive loss is as high as about 0.2 dB over the full range of 50 percent to 80-odd percent of split ratio as shown in FIG. 15.
In contrast, the prior art optical splitter shown in FIG. 16 has the drawback that the split excessive loss becomes high as the split ratio increases as shown in FIG. 17. For this reason, there is a limit to the increase in the split ratio.